1. Field of the Invention
The present invention relates to a thermosetting resin composition and to a prepreg and a laminated sheet which use the resin composition. Particularly it relates to a laminate material suitable for a multilayer printed circuit board which is excellent in flame resistance, heat resistance and electrical properties and shows a low shrinkage percentage in curing.
2. Description of the Prior Art
Laminate materials hitherto mainly used for multilayer printed circuit boards are phenol resins, epoxy resins, polyimide resins and the like to which brominated modified resins or additive-type flame retarders have been added, to impart them flame resistance. Particularly for large-scaled computers, for which an increasingly higher density has been desired, polyimide-type resins which are excellent in heat resistance and dimensional stability have been used. In recent years, however, with increase in the processing speed of large-scaled computers, printed circuit boards of low dielectric constant have come to be required to increase the signal transmission rate. To serve as such laminate materials of a low dielectric constant, there have been developed laminated sheets comprising polytetrafluoroethylene (PTFE) resin or polybutadiene resin. With regard to laminated sheets of this kind, mention may be made, for example, of Proc. NEPCON (1981), pp. 160-169 and Japanese Patent Application Kokai (Laid-Open) No. 126,451/80.
Further, Japanese Patent Application No. 33,050/86 discloses a flame resistant resin composition of a low dielectric constant which comprises a poly(p-hydroxystyrene) derivative and an epoxidized polybutadiene.
However, PTFE laminated sheets have a high thermal expansion coefficient and hence an insufficient dimensional stability at high temperatures because PTFE resin is thermoplastic and has a low glass transition temperature. Particularly, they show an unsatisfactory through-hole reliability in multilayer bonding. Further, PTFE is generally bonded by means of heat melting pressure bonding owing to lack of suitable solvents, so that its very high melting temperature is disadvantageous. On the other hand, polybutadiene resin has a defect of being easily combustible owing to its molecular structure, so that it must be incorporated, to be imparted flame resistance, with additive-type flame retarders such as decabromodiphenyl ether and tryphenyl phosphate or reactive flame retarders such as tribromophenyl methacrylate and tribromophenyl acrylate. The incorporation of these retarders raises the problem of deteriorating the good electrical properties, heat resistance and dimensional stability which are inherent in polybutadiene resin.